Anything and Everything you ever wanted to know about the 707.
Author: David Hingtgen
Submitted by: David Hingtgen Date: 02-29-2004
Comments: (13)

Ultimate Boeing 707 Guide

Important note: For this article, I will
be referring to each and every variant in as complete a designation as possible.
Thus "707-120" MEANS 707-120, and -120 only. Not the -120B, not the extended
family of the -120. This ESPECIALLY goes for the -320. -320 means -320, not
-320B, not -320BAdv, not a -320C. -320 variants are the most misunderstood thing
about the 707. Example: AA had no -320's. Now people will say "Hey, there's
LOTS of pics of AA -320's". Well, those are all -320B's, -320BAdv, and -320C's.
Not -320's. I will never use "-320" to indicate the entire -320 family. Where
something is common to the variants, I'll go something like "-320/B/BAdv/C",
thus indicating the -320, -320B, -320BAdv, and -320C. Or more commonly "-320BAdv/C"
for they are nigh-identical. It would be a LOT easier for us 707 fans if Boeing
had (retroactively) called the original version of the -320 the -320A, as it
makes a LOT of sense (since the next ones were the -320B and the -320C), and
it would eliminate lots of confusion.

Also, many pics of the early 707/720's are pre-delivery, or first few months
of service. Therefore, they will be different than the "definitive/standard"
version. This is especially true for the 707-220. So don't count on them for
being representative of their type.
Now on to the guide! (PS--I believe this will cover the variants of the 707
family in more depth than any article in any magazine, or even the best 707
book. They're fine for why things happened, who ordered what etc, but they gloss
over the actual differences between the models. And even drawings are often
wrong--any 1/144 modeler can tell you that. While checking out 720B horizontal
stabilizers (one of the last things I researched) I did find the "official"
3-view drawing from Boeing, used in nearly every magazine and book, and at Boeing.com,
and their handy little "Airplane characteristics for Airport Managers"--well,
the *numbers* in the last one was right, but every drawing was wrong).
A note before the rest of the guide: one of the most obvious features of the
707 family is the HF antenna on top of the fin. However, not all of them had
them. Just about every -320B/C you'll see will have it, but there's quite a
few 720/-120B's and such that don't. It's roughly equivalent to whether it's
an over-water plane or not. (Thus domestic 720's for AA don't have them while
Aer Lingus' 720's do)
Despite what it says on the tail, and being N70700, this is not the first true
707:

THIS is the first 707:N708PA:

The 367-80 is not a true 707. Looks like one, but isn't. Different fuselage,
wings, engines, and tail. Basically everything is subtly different from a production
707. The above pic is the first 707---a lot of people don't know what the 707's
house colors were---not rust/yellow, it's white with red and blue pinstripes,
like the current colors.

707-120

Anyways, note the engines. Those are Pratt and Whitney JT3C's. They are nothing
more than the military J57, used in the B-52 Stratofortress and F-8 Crusader,
among others. (And the F-8 gets an afterburner--imagine one (or four!) of those
on a 707). Unlike modern passenger jet engines, they are not equipped to provide
air for cabin pressurization. (The first engine designed from the start to do
so was the JT8D). So for cabin air purposes, you need one of the key 707 spotting
features--turbocompressors. (Or more accurately, the turbocompressor hump).
The turbocompressor is basically a little engine-driven turbine that sits right
above the engine's fan casing, just ahead of the pylon. For obvious reasons,
Boeing put a nice little fairing over the turbocompressor (t/c or T/C from now
on) and faired it into the pylon. But it leaves an obvious "hump" on top of
the engine just in front of the pylon. In the above photo, there's one on every
visible engine (and the not-visible ones too, or at least there should be).
Location/number of t/c humps is a good way to tell 707's apart, but beware of
odd-balls like AA. I'll note it when appropriate. Side note: a t/c hump doesn't
necessarily mean there's a t/c in there. Weird, but true. For example, Air India
707-420's. Like all 707-420's, they have 4 t/c humps. BUT Air India -420's only
have 2 t/c's. The other two are empty humps! (Engines 2 and 3 have them, 1 and
4 don't) :) (And of course, just to be different, AA's -320B's etc have only
2 t/c's, and only have 2 humps)
Might as well put in "the big pic" now, as the entire rest of the article refers
to it. This shows the four different 707 wings, and lists who uses what wing.

Ok, just look at the top one. That's
the wing used on the 707-120, 707-138, and 707-220. Note that the leading edge
is straight, and so is the trailing edge (except for where it curves to meet
the fuselage). That's a key -120 feature--straight leading AND trailing edges
on the wing.

Next feature----big ventral fin under
the rear fuselage Not installed until later (as in 1 year or so after entry
into service), and not ALL -120's got them (though I'd say it's like 99%--never
found one post-1960 that didn't)

If a 707-120 has a ventral fin (and they
all did, eventually), it'll be the big one. Early 707's (as in most) had poor
yaw stability--rudder wasn't hydraulically powered, yaw damper ineffective,
everything. So the vertical stabilizer was extended 40 inches, and a ventral
fin was installed. (This is why many early 707's have "short" tailfins--they
just hadn't gotten the taller fin yet--but they ALL got them later on. Also,
it's quite possible to have the HF antenna on top of the fin, yet still have
the short fin--see CO's -120's early on)

Another 707 feature--- a fillet-flap. It's actually part of the trailing edge
fillet/fairing, and it splits top/bottom. The top part stays as it is, but the
bottom pivots down. This is common to the 707-120/138/220/720/720B/120B/138B/320/420.
(The -320B and later will introduce a new style)

Finally, 707-120's have leading edge flaps (Kr?ger flaps) inboard of the outboard
engines. (AKA just inside of the #1 and #4 engines) Two sections only, per wing.
(Scroll down to the -138 pic to see them well)
In summary: 707-120 has 4 t/c humps, straight leading and trailing edges, and
a big ventral fin. Only ordered by AA, TWA, Air France, and Pan Am, and 2 ordered
by Cubana which went to Western. Max speed, M=0.884

707-138

The infamous "short" 707 only ordered by QANTAS. 10 feet shorter than
the -120. Since 707's are built with 20-inch frames, that means it's 120 inches/20=6
frames shorter than a 707-120. Using the tried-and-true "window-counting
method" you get 3 frames removed from ahead of the wing, 3 frames behind. 10
feet shorter=more range. Same idea as a 747SP. Only physical difference
from the -120. (Well, the only guaranteed difference- any particular -138 or
-138B may or may not have an antenna projecting forward from the right wingtip--depends
on what year, what plane, and planetary alignment) Side note: they were the
first type of 707 to be certified to carry a spare engine. (Should be SLIGHTLY
faster than a -120, but not noted anywhere I could find) Has all 4 t/c humps.
Here's a -138 pic, early into service. No "tall" (40 inches FYI) rudder/stab
yet, and no ventral fin--so it's got to be 1959, maybe early 1960. Note the
spare engine right by the fuselage, and how the entire wing has but 2 little
kruger flaps, inboard of the outboard engine. (All -120/138/220 krugers are
like that)

707-220

Braniff's (and only Braniff's, though BWIA bought some used) version of the
-120. Same as a -120, but with JT4A engines for more power. See the above pic--that's
a Braniff 707-220. Big ventral fin, straight leading and trailing edges, just
like a -120. Just different engines. How to tell JT4A's from JT3C's? The exhaust
is a bit different, with fewer/shorter/stockier "tubes". Same engine as a 707-320,
easier to find pics of those. Max speed M=0.884

720

The other short 707. :) It is BASICALLY a 707-120 that is shortened by 8 feet,
4 inches. (100 inches/20=5 frames) Window-counting gets you 1 frame removed
from ahead of the wing, 4 frames removed aft(compared to a 707-120). An interesting
note: though a 720 is only 20 inches longer than a 707-138, it is not simply
one frame longer than a -138. Compared to a 707-138, It actually has 2 frames
added ahead of the wing, but 1 frame removed aft of the wing. The net effect
is 1 frame longer, but it has different proportions than if you simply took
out one frame. You'll note a -138 has its forward baggage door noticeably further
aft than a 720 does: a -138's door actually contacts the wing/body fairing leading
edge, while a 720's door is well forward of it. But it's got a lot more changes
than that. First (because it's quick and easy to write) is that the main gear
wheels/tires are smaller. 40 inches vs 46. Sorry, no photo, you'll just have
to believe me. (As if I could find a photo of a 720 tire with a ruler for scale....)

Also, a small ventral fin: Only half the size of a -120's, very easy to tell
big from small.
Also, t/c humps only on engines #2 and #3. (The inboard ones). See photo above,
and below.

Next, Kr?ger flaps inboard of the inboard
engines---just visible behind the #3 pylon in the pic above. 2 segments per
wing. Also, Kr?ger flaps outboard of the outboard engines, 4 segments per wing.
Pretty much a continuous line of flaps now across the leading edge, with a few
small gaps. (Outboard of inboard engines, and extreme inboard leading edge)
And finally, the BIG 720 change--the infamous wing glove. Scroll back up to
the big wing diagram and look at the 3rd one. That's the 720 wing, used on the
720, 720B, 707-120B, and 707-138B. It is the basic -120 wing with the glove
tacked on to the leading edge. Note the glove does not SMOOTHLY attach, there's
a noticeable kink right at the inboard pylon--much easier to see if you tilt
your head. (Tilt your head to the side whenever you're looking for a kink in
the wings--trust me, it makes it a LOT easier to see them) The diagram is not
100% correct--the glove does not merely add to the front of the wing. It WRAPS
around and above, all the way back to the front spar. The wing is THICKER as
well due to this (well, the front half is).

(Also, the glove provides room for the new kruger flaps, mentioned above, and
makes the plane faster, Max speed M=0.906) At least one UA 720 seems to have
never gotten a ventral fin, but nearly every other 720 did. Ordered by UA, AA,
EA, BN, Aer Lingus, Pacific Northern.
In summary: 720's are a short 707-120 with smaller wheels, a small ventral fin,
t/c on #2 and #3 engines only, and the wing glove.

720B

This is a big important "milestone" aircraft. It is the basis for all other
"B" aircraft (despite the -120B flying first--kind of like how the 767 actually
came out before the 757). Anyways, lots of changes for the 720B. First and obviously,
turbofan engines. JT3D's to be exact:

Also note the outboard pylons---the upper surface is a nice straight angle.
Compare to the outboard pylons of the PNA and Aer Lingus 720's above which have
a slight bend in them near the front. You can also see the kruger flaps in the
wing glove a bit easier in this pic.
Next, and hardest to see/show is the horizontal stabilizer extension. If you
look at any 707 we've discussed up to this point, they all have the same stabs.
And the elevator extends all the way to the tip. On the 720B, a 20 inch extension
is added to each tip. But it is a solid, non-moving piece. The elevator is not
extended. Thus, the elevator no longer goes to the tip, and is easy to spot
when they're up. Compare these two pics--the top is an EA 720's stab, identical
to a -120/138/220, and the lower one is a NW 720B's, also used in the -120B/138B.

707-120B

Now this is one heck of a change. Basically, you take a 707-120, and apply nearly
every change from the 720 and 720B combined. You end up with a "720B stretch".
So to go from 707-120 to 707-120B you do this:
Add wing glove, replace big ventral fin with small one, add flaps inboard of
the inboard engines, add flaps outboard of the outboard engines, give it JT3D's,
add the stab extensions, remove the #1 and #4 t/c's and give it 720B-style pylons,
and you get this:

Max speed M=0.90. Only ordered new by AA and TWA, all others converted from
-120's. The last few newly built ones ordered by AA and TWA had the later-style
nose-gear doors. N6721 for example.

707-138B

John Travolta's plane, and probably the most-photographed 707 in the world by
now. Much like a 707-120 being converted into a -120B got all of the 720/720B
changes, so did the 707-138 converted to a 707-138B. (Of course, just like the
707-120B, some were built as a "B" and not converted from earlier versions).
Stab extension, wing glove, kruger flaps, etc. So you end up with a plane very
similar to a 720B, just 20 inches shorter and a TC hump on #4.

Notes:

1. The -138 came into service later than the -120, and I can find NO pics of
a -138 with a ventral fin and the taller tail fin. It is possible that they
went straight to being -138B's, without a new tail fin or ventral fin. Thus,
the -138 might NOT have a big ventral fin, because by the time they would have
gotten them, they became -138B's instead, and got a small fin. A pic of a -138
with a ventral fin (and/or tall tail fin) would be appreciated

2.
Like the -120B, Some of the very last ones got -320C style nose gear doors.
Here's a pic of the "standard" 707 nose gear doors:

See that tiny little sliver just behind the strut? That's the nose gear door.
That's the nose gear door for the 707-120/120B/138/720/720B/220/320/320B/420.
But not the last few -138B's. :) Those -138B's have the much later style rectangular
nose gear doors, depicted below in the -320 family pics.
In summary: a 707-120B that's short, possibly with new nose-gear doors. Has
no #1 t/c, but does have the #4 t/c (unlike -120B/720B). Fastest of all 707's,
max speed M=0.91. (Speed info comes from the FAA type certificates, BTW--they're
a great source of info for some things)
Well, that's it for the "standard" 707 family, now we move into the ones that
actually get modeled in the 1/400 world, the stretched 707's.

707-320

Actually the second-oldest 707. Predates the 720 and 707-220. PA and TW had
these in service by 1959.

Things to note: it's longer than a -120 etc, has a big ventral fin, t/c on all
4 engines, JT4A's like a -220, and practically an all-new wing. The wing's the
big main difference. Go WAY back up to the wing diagram, and look at the second
one. That's the -320/420 wing, exclusive to those two. It is highly-modified
-120 wing. (Look closely at the panel lines on the center section). It's got
an inboard plug, an outboard plug, and a revised inboard trailing edge. See
the kink? That's where it starts to angle back a bit to increase the inboard
area even more than it would with just a plug. Also note the engines are all
further outboard than the -120's (and any 120/220/720 variant etc). Leading-edge
flap arrangement is the same as a -120/220---only a pair inboard of the outboard
engines on each wing.. Also has a fillet-flap right in the wing trailing edge
fairing like the shorter 707's/720's. LE flap arrangement:

Next, stabilizers. Significantly larger than any of the ones discussed before.
But it's basically just a scaled-up version of the -120's. Same proportions
and all, just bigger--45ft, 8in span as opposed to the 1-20/220 span of 39ft,
8in. Elevators still go all the way to the tip. Still has the original nose-gear
doors.

Finally, it's got a little fairing on the belly, right between the leading-edge
tips of the wing/body fairing. Kind of a forward-pointing triangle. Makes the
belly bulge ever so slightly when you look at the plane from the side--look
at all the 320-family pics here--it's visible most of the time---it's not that
the wing protrudes below the fuse, the belly's a bit fatter. Here's a pic of
an Iraqi -320C's belly, but the ENTIRE 320/420 family has this fairing--also
note there's a little forward-pointing triangular NACA duct in the fairing,
too.

Max speed M=0.887 Only ordered by PAA, Air France, TWA, Sabena, South African.

707-420

This one's quick and easy to do. It's a -320 with RR engines and an HF antenna
on the right wingtip. And since the antenna's there, the little green light
has to be moved back a bit, to the side of the tip of the wing, rather than
the right-front corner. Ordered by BOAC (and subsidiaries etc), Varig, El Al,
LH, Cunard, and Air India. A few very late ones had the late/320C-style nose-gear
doors, but all others had standard -120/220/320 style doors. Here's a -420:

Good 707-420 engines (RR Conway 508) pic:

Max speed M=0.852

707-320B

Turbofans (JT3D's) on the -320. But much more than that. The "standard" t/c
configuration is on engines 2, 3, and 4, but AA just went with them on number
2 and 3 only. Has a drastically modified wing compared to the -320. Go back
up to the wing diagram, it's the bottom one. The trailing edge is modified AGAIN.
Another kink! The inboard part of the inboard section has had it's sweep decreased
yet again, and now it intersects the fuselage at a perfect 90-degree angle.
(It's much easier to see the inboard-most kink if you tilt your head). No more
nice curved fairing, no more fillet flap--instead, a flipper-flap! See here:

Also note how the flap actually attaches to the side of the fuselage, like how
a DC-10's does (but even more so).
The leading edge is changed as well. The leading edge was extended slightly
at the inboard pylon, thus creating a dog toothed joint. Here's a pic showing
it:

The -320B also adds a lot more Kr?ger
flaps to the leading edge---they now extend all the way from the inboard pylon
to the wing-tip. Plus, there's a bit inboard of the inboard pylons. It only
spans maybe 1/3 the distance from the pylon to the fuselage. See it and the
other flaps here:
-320B left wing leading edge, showing flaps entirely along LE from inboard pylon
to wingtip, and short LE flap inboard of inboard engine.

Finally, it's got a little flare at the wingtip that adds 3 feet to the overall
span. Refer back to the main wing comparison drawing. See how all other 707-family
wings have blunt tips? But the -320B has kind of a triangular/curved section
added on at the very end.. Max speed M=0.887
Just your average 707-320B.

707-320C

The -320C was next, not the -320BAdv. The -320C has the new, rectangular style
nose gear doors. (Visible in the Varig pic below). And of course it has a cargo
door. Also has a hat-rack door just behind the wing. Hat-rack door you ask?
Well it's not as tall as a full door, but bigger than an over-wing emergency
exit. It's as tall as the hat-rack they had in old jets, like say, 707's. It's
required because if you put cargo up front, the front exits are blocked off,
so the passengers in back need a way out. Also strengthening for the floor,
gear, wings, etc. Little things. Also, more Kr?ger flaps! (Boeing just loves
to add Kr?ger flaps). Added two more segments inboard of the inboard engines,
which now span a large portion of the pylon-to-fuselage area. (They are very
close together, and it's often hard to see 3 distinct segments)

The easiest thing to see is that the -320C has no ventral fin. That's important.
You know that little flare the -320B has on the wingtip? Well that, combined
with the -320C's extra kruger flaps, actually solved some aerodynamic problems,
so the ventral fin could now be removed.
In the following pic, there's several things to note. First, the two small pale
square markings just below the cheatline, directly under the word "Brasil".
Those indicate the presence of a cargo door. It's almost impossible to see the
cargo door sometimes (like in this pic) but those little squares are a very
useful indicator. Secondly, the hat-rack door right behind the wing. It's bigger
than the over-wing exits (see how the bottom edge is lower, it's as low as the
rear door's) but it's only as tall as the emergency exits. So it's about a 3/4
size door.

Average 707-320C. Note the lack of a ventral fin, as well as how the inboard-most
flaps extend much further inboard than those of the -320B. Final note: 707-320C's
built for pure cargo services (notably some of AA's fleet) lack both windows
and hat-rack doors. AA's also lack the #4 t/c hump. (AA's -320BAdv's also lack
the #4 hump)

707-320B Adv

Yes, Adv for "Advanced" like a 727-200Advanced.
This is a -320B with all the changes the -320C has, internal and external (loss
of ventral fin, 3rd LE segment inboard, etc). Early (1st year) -320BAdv's still
had the old-style nose gear doors, but most have the later rectangular ones.
No cargo-door (duh), no hat-rack door. But otherwise identical to a -320C.

Note how the inboard-most Kr?ger flap segments cover the majority of the span
from the inboard pylon to the fuselage---all -320BAdv/320C's have this, the
-320B only has a small segment. Triangular belly fairing bulge also visible.

Finally, with all those 707-320B variants, a lot of things could be, and were,
retrofitted to earlier versions. Here's a -320B with the new nose gear doors,
like a -320BAdv.

You'll also find -320B's with the added kruger flaps of a -320BAdv/C, and/or
removal of the ventral fin---like Air France has done on F-BHSV, one of the
first -320B's built:

This early non-Adv -320B has been upgraded with some -320BAdv features like
the additional inboard LE flaps and removal of the ventral fin. Note that it
still retains the original early-style nose-gear doors.

That's it for the main guide, now for a few miscellaneous 707 notes.
There are two types of secondary intake doors on the JT3D's cowling. The earlier
style is like this:
Small, vertical, thin rectangles.
The later style is like this:

Larger, square doors.
Plenty of places say the new style is for the -320BAdv (and hence -320C), but
that's not true from what I can see. I've found both old and new style on -320B's,
-320BAdv's, and -320C's. I would certainly say most -320C's have the new style,
but nothing more specific.
Finally, the true freaks of the 707 family. Northwest's 707-351B(SCD)'s. Here's
a pic:

Well, it's got a cargo door. That'd make it a -320C to most people. But it's
not. The floor is not strengthened, so it can't carry much cargo. It also doesn't
have a hat-rack door, (not visible in this pic), so it's not a -320C for that
reason too. So is it a -320BAdv with a cargo door? Nope, it doesn't have the
inboard-most kruger flaps (also not visible, but I trust the people I learned
about this from), which all -320BAdv/320C's have. And also note--old-style nose-gear
doors. (The rectangular ones you see are actually the gear-well doors opened,
note they're FORWARD of the gear doors---you rarely see these open except during
gear movement---the actual gear door is the tiny little one behind the strut,
like on early 707's). Basically, it's a -320B with a cargo door, and no ventral
fin. (Any non-Adv-320B without a ventral fin had it removed after being built---but
these non-Adv320B's were built without them). It does not have any of the strengthening
applied to the -320BAdv/320C (floor, wings, landing gear), nor the kruger flaps
of them. But the lack of a ventral fin is a very "Advanced/C" feature. So what
is it exactly? It's a 707-351B(SCD), the total freak of the 707 family. Thankfully,
there were only 5 of these misfits built. (N351US to N355US).

Credits:
Lots of people's postings on the web and e-lists and such, books, etc, but most
notable are DC-8 FANJET, panam707, and Impman from Diecastaircraft.com, Clint
Groves, Jeff Jarvis, and Jodie Peeler from Airliners Modeling Digest, Andrew
Klein (for the 720 pylons), "707 and AWACS, Detail and Scale #23", "707: Pioneer
Jetliner", Airliners issues on the 707, 720, and John Travolta's -138B, hours
at Airliners.net and images.google.com, and just plain paying attention whenever
someone mentions something about a little 707 detail!

Good article, having been "assigned" an ex-AA 707-323B to maintain parts and
make sure the log book stayed clear. One thing though, the 720 was actually
called a 707-020, designed to compete with the Convair 880, and having short
runway capability with really spectacular take off performance. However the
extensive modifications were such it was a "different animal" than any other
707 model. United, who had committed to a fleet of DC-8s had ordered the
(then) 707-020, and so not to confuse people, they called them 720's. Boeing
adopted the name. If you dont believe me, ask Boeing, thats how I found out.

Oh yeah. The tall vertical stabilizer, along with the small ventral fin in the
rear was mandated by then BOAC for the 707-436 (Conway Turbojet). Boeing went on to add this feature to production A/C and then those in service were retrofitted. Some were, some wern't and some of the ventrals were bigger than
others etc. Another diddy from Boeing.

The reason that the 707 was different than the 367-80 was due to the airlines
need to have six abreast seating. And the capacity needed. The dash 80 wasnt wide or long enough. They were competing against Douglas and the then model 1881
(DC-8) which was being configured to suit the airlines needs. Boeing was converting a former USAF aircraft for civil use, and had to modify to compete.
This also raised a problem with airlines trying to keep the A/C interiors uniform. Seats,bulkheads,PSU's and other interior cabin components were not
interchangeable between Boeing and Douglas aircraft. Maybe thats why some airlines flew all Boeing jets. Only one US airline flew both 707 and DC-8
Pan Am.

Allow me to add just one very minor addition to the list of changes between the 707-320/420 Intercontinentals and the 320B/C machines: the wing/body fairing of the later machine was modified aft of the rear emergency exits. This was in order to accommodate the "elephant's ear" or "flipper" flap, whose inboard part swept along the body sidewall. The fairing was made marginally taller. The difference is fairly visible when comparing rear three-quarter photos of earlier and late 707s.

Im Back, you know theres one thing that isnt covered here was "Where the heck did the name "707" come from.
As per Boeing.
Following a new model designators developed after WWII, the 300/400 were retained (thus the 367-80 and the B-52 which was the model 454), the 500 was for Turbine Engines, the 600 for Spacecraft and 700 for Jet Transports.
The name 707 had a better ring to it, simple...

Well....having tinkered with several versions of the 707, I freely admit to not having done all the mods....eg I have a British Caledonian -320C (pic in the galleries) but didnt get round to dealing with the wingtips - still can and will one day. Sure there is a lot else wrong with it so it wont satisfy the purist, but it'll do.
Likewise most of my 707 models.
Great piece david - thanks

Given all nuances of the myriad of 707 variants, this is MUST READ article for anyone serious about the accuracy of their 707 model! This has been my "go to" source while working on my 707s. Well done, David- I commend you on your research